I saw something that said the reason the universe exists is that everything exists, in an infinite multiverse. This then answers why the laws of physics of our universe are the way they are, which is that they are one of an infinite set picked by chance, and removes the arbitrariness of our existence. However the idea of everything existing is hard for me to even begin to comprehend, and it seems like there might be some paradoxes that arise from it. To be clear I am asking if the concept of "everything" is logically coherent.
Is the concept of "everything" logically coherent?
I would say largely yes, but within the framework of your views on ontology. Therefore, your "everything" and someone else's "everything" may not agree in definition.
I saw something that said the reason the universe exists is that everything exists, in an infinite multiverse.
Yes, there are many claims to why the universe exists. The scientific study of the origin of the universe is known as cosmogony. Of course, there are non-scientific explanations too. Origin myths are nearly universal in the mythologies of the world, and creation myths seem to serve a psychological purpose. So, when you start talking with certainty about the origins of the universe, you more likely than not are engaged in the metaphysical speculation. From the standpoint of empirical evidence, claims about the origin of the physical universe are largely those made in regards to the Big Bang theory. The Multiverse hypothesis is somewhat controversial in that such discussions seem to be largely non- or pseudo-scientific. Remember, creation myths seems to important psychologically regardless of their actual veracity. My personal views are solidly with physicist Sabine Hofstetter who claims that multiverse theories are religions, not scientific theories. What is everything? What is the universe? There is much philosophical contention over everything that exists, because it is not agreed upon what "to exist" means.
But let's talk language. First and foremost, the idea of "everything" is a linguistic artifact. Utterances are nothing more than elements of sound production. Propositions like definitions are conceptual representations, building blocks of meanings, the study of which is known as semantics. How can one define everything? Well, in a naive sense, just like MW:
Definition of everything
1a : all that exists
b : all that relates to the subject
2 : all that is important you mean everything to me
3 : all sorts of other things —used to indicate related but unspecified events, facts, or conditions
Right away, the 1a leads us to the most philosophical sense, "all that exists", which then devolves into a discussion about "existence" and hence why ontological discourse is important (and dare we say exists?) Since there are posts about existence on this site, we won't rehash, however, we can give a few pointers to philosophers who are well regarded in metaontological circles.
First, there is Aristotle whose works are voluminous. Aristotle is not the only Ancient Greek of repute, but he is rather indisputably the most influential by way of the Scholastics domination of philosophy during the growth of the Catholic Church which accoring to WP:
The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.3 billion baptised Catholics worldwide as of 2019.4 As the world's oldest and largest continuously functioning international institution,8 it has played a prominent role in the history and development of Western civilisation.
Aristotle, however, came after Plato. Plato is tremendously significant because of his theory of Forms. As a mathematical constuctivist, I personally rail against Platonic thinking, however, no reasonable thinker can doubt the power of the idea, and the way it continues to captivate philosophical thinkers.
I would include Immanuel Kant because he is often considered as the progenitor of modern philosophy and is a father both to analytical and Contintental philosophy. Kant challenged the nature of "existence" when he spoke of what are now known as Kantian forms (Britannica) and advocated the thing-in-itself.
More contemporaneously are the Austrian Meinong to whom the phrase Meinong's jungle refers. Meinong was influential because he systematically attempted to make sense of non-physical referents. According to WP:
The Meinongian theory of objects (Gegenstandstheorie) was influential in the debate over sense and reference between Gottlob Frege and Bertrand Russell which led to the establishment of analytic philosophy and contemporary philosophy of language. Russell's theory of descriptions, in the words of P. M. S. Hacker, enables him to "thin out the luxuriant Meinongian jungle of entities (such as the round square), which, it had appeared, must in some sense subsist in order to be talked about".3 According to the theory of descriptions, speakers are not committed to asserting the existence of referents for the names they use.
Rudolf Carnap is seminal because of his position among the logical positivists, who in analytical circles, who collectively were tremendously influential philosophical thinkers when it came to science. Carnap's approach to existence was very much about eliminating grand metaphysical speculation, and attempting to reduce existence to observation, particularly the scientific kind.
And lastly, I'd include Quine, who is larger than life when it comes to contributions in logic and philosophy and influenced a generation of thinkers with his views on existence and the naturalized epistemology.
Now, what I would argue is that the linguistic device known as "everything" is essentially a psycological operation of the mind in a way that the antipsychologists would have decried. In the philosophy of mathematics, foundational to understanding math is set theory, and in set theory we have the notion of the domain of discourse, the union operation, and the universal set. It is natural for the mind to identify the existence of things, then to differentiate and keep count of things. Subitization is built into the brain.
So, is "everything" logical? Yes, given your logic which determines what does and doesn't exist, and your system of keeping track of individual things. Whether that system is the one proposed by Plato, Meinong, or Quine is another matter entirely.
Firstly, we don’t know that there’s an infinite multiverse. If there is then the probability of any given thing existing approximates to 1 but mutually contradictory things can’t exist in the same frame of reference. For example, an irresistible force could exist and an immovable object could exist but they couldn’t have the same frame of reference. In everyday terms you could have a force that can move anything north or south and an object that couldn’t be moved east or west. So if you want an irresistible force and an immovable object then you can have both but only in such a way that they can never meet.
To the question, "What is there?" Quine 'famously' replied, "Everything." And I think Russell spoke of the robustness of existence, as something that ruled out things that 'there are some of' that, yet, don't exist.
But the fact that unrestricted quantification is relatively uncommon is no reason to doubt it is attainable in certain contexts. Unfortunately, many philosophers have recently doubted that genuinely unrestricted quantification is even coherent, much less attainable.21
21For examples, the reader may consult Dummett 1991; Fine 2006; Glanzberg 2004; Hellman 2006; Lavine 2006; and Parsons 2006.
Quine's set theory has a universal set, but so resistance to talk of a universal set is a motivating factor in at least some rejection of unrestricted quantification.
No, because you've misunderstood and misrepresented the concept
A multiverse doesn't mean "everything can exist". It means that universes can exist with all possible values of underlying basic physical constants. The maths shows (apparently; this isn't my field) that for many possible values of those constants, the result would be that universe ceasing to exist very quickly. Only certain ranges of values give stable universes.
That doesn't mean everything will exist though. Changing basic physical properties doesn't mean you've made anything particular happen.
To find out what is everything we must first ascertain what is a thing? You must question your own epistemology. Are you naturalist or are you a supernaturalist? This has great bearing on what you consider to be a thing. Do you allow for extant immaterial things? God is immaterial but to Christians he is still a thing. God is not nothing.
For some people in the scientific community the idea of a thing being immaterial and in existence is far beyond anything there worldview could ever consider possible. Do you allow for something that exist in a way that is not bound to our physical universe?
These are the questions you are starting your philosophical journey with. Enjoy your journey you are about to start questioning every cherished belief a person can have.
Could everything exist?
Logically speaking, possibly. But if everything is that which is existing then the statement is just a tautology: a thing is that which exists, so it follows that all things are neccessarily existing. By this, I don't mean it's existence is necessary, it could be contingent.
To get out of this tautology we need to broaden what we mean by existing. It could mean everything that has ever existed. In which case the answer is no. There are no dinosaurs on earth. They don't exist.
But they could concievably exist, say for example we could find a full genetic code for a dinosaur and we had the technology to 'engineer' such an animal from its code. But whilst we might be able to resurrect an extinct genus or an extinct species, we cannot resurrect a specific individual of that species. Take Genghis Khan, even were we to build him, it would not be him. A person is much more than his genetic code. He is also his world, his culture and his language.
However, the world you are considering is much larger. It is all concievable worlds as modelled by our physical laws. This again, though, is not everything possible. Unfortunately, its a highly speculative idea with very little chance to no chance of ever being proven, even in principle. But its very dramatic, which is why everyone knows about it.
Apparently, late medieval theologians contemplated how many angels could dance on the head of a pin. A hundred? A thousand? A million? It's a byword in wasting time debating nonsense today. Likewise, one could ask certain modern physicists how many universes can you fit into a multiverse? A trillion, a gadzillion, a googleplex - maybe even aleph one - with probably just as much eventual sense (ie. nonsense).